Alpine Head unit (9812) what cliosportcraig suggested - £240
Focal front componants - £150-£220
Directed or JL Sub (Studio or W3 Series Subwoofer)£150
Genesis, PG, Rocky, Alpine Amps.... You can either get a 4 channel amp & use a channel for fronts & bridge ur sub on the other.... Or you can spend a lil extra (maybe a hundred?) & get a dedicated 2 channel amp & a monoblock for the sub. Would be better.
Not sure that there is many wiring interfaces out for the screen, but like cliosportcraig said there is loadsa wiring interfaces for the steering stalk, but trust me, If you can get an alpine head unit & keep the stalk, it is MUCH better to do that than get a sony head unit & have the screen. sacrifice the screen, you will not regret it.
I might be selling my PG QX900.d monoblock & Directed Studio 12" sub soon..... just thought id mention that hehe.
P.S You might wanna try going around ur local ice shops seeing if any of there amps that are on display they will do a deal on, with a 12 month warrenty, they will usually drop 20%-25% off the price just coz its a display one, I got £200 off my front speakers, & still got all the manuals, boxes & even a 12 month warrenty!